1. Field of the Invention
The present invention relates to an inkjet pigment ink that provides a recording product with an excellent abrasion resistance even in the case of using glossy paper smoothed with a surface coat, while having excellent ejection stability from an inkjet nozzle and
2. Description of the Related Art
An inkjet recording method performs a recording operation such that small droplets of a recording liquid (ink) are caused to fly to be attached onto a recording medium, such as paper. As an ink ejection means, there is known, for example, a method of ejecting a liquid droplet involving imparting thermal energy to the ink using an electro-thermal converter as means for supplying ejection energy (thermal inkjet method) to generate an air bubble in the ink. According to the method described above, in particular, a recording head having high-density multi-orifices can be realized, thereby being capable of recording (printing or producing) an image with high resolution and high quality at high speed (Japanese Patent Publication No. S61-59911).
On the other hand, as the coloring materials to be contained in the inks, dyes, which dissolve in ink solvents, are conventionally mainly used. However, from the viewpoints of light resistance and water resistance, the coloring materials made of pigments are now being investigated. The pigment inks are sometimes inferior to the dye inks in abrasion resistance, because the pigment inks perform the printing by attaching pigment onto a surface of the recording medium. In addition, it is pointed out that the pigment inks lose their glossiness depending on the kinds of the recording media to be used and the particle diameters of pigment.
Heretofore, the addition of a resin to the pigment ink has been examined to improve the abrasion resistance of an image. In other words, this method intends to fix the pigment onto the surface of the recording medium by the resin to be added in the ink. However, when the resin is made of a water-soluble polymer, the addition of a sufficient amount of the resin for fixation of the pigment to the ink leads to an increase in viscosity of the ink, thereby resulting in, for example, poor ejection stability. In contrast, Japanese Patent Publication No. S62-001426, Japanese Patent Application Laid-Open No. S55-157668, Japanese Patent Application Laid-Open No. H03-160068, and Japanese Patent Application Laid-Open No. H04-18427 proposes the addition of a water-soluble resin emulsion to the pigment ink to decrease the viscosity of the ink and to improve ejection characteristics. However, after the printing, the pigment and the resin emulsion are independently present on the surface of the recording medium, so the resin emulsion can be hardly effective to fix the pigment, thereby resulting in poor abrasion resistance of the ink.
In contrast to this, an ink set for improving the abrasion resistance and the glossiness is proposed as described below (Japanese Patent Application Laid-Open No. 2002-30235). The document discloses that the ink contains a pigment and fine particles of a thermoplastic resin with a glass transition temperature of from 50° C. to 150° C. The resulting image after the printing is heated at the glass transition temperature of the resin fine particles or more to melt the resin particles, thereby combining the pigment and the resin components. More specifically, the resin fine particles are molten to form a film, and the pigment and the resin components are then combined in the film to fix the pigment on the surface of the recording medium, thereby improving the abrasion resistance of the ink. However, the technology requires a step of heating in the process of image formation. Therefore, more excellent ink in respect of clogging resistance and storage stability is desired.